1. Field
This disclosure is generally related to a passive optical network (PON). More specifically, this disclosure is related to a PON with extended reach.
2. Related Art
In order to keep pace with increasing Internet traffic, network operators have widely deployed optical fibers and optical transmission equipment, substantially increasing the capacity of backbone networks. A corresponding increase in access network capacity, however, has not matched this increase in backbone network capacity. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks still presents a severe bottleneck in delivering large bandwidth to end users.
Among different competing technologies, passive optical networks (PONs) are one of the best candidates for next-generation access networks. With the large bandwidth of optical fibers, PONs can accommodate broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, PONs can be built with existing protocols, such as Ethernet and ATM, which facilitate interoperability between PONs and other network equipment.
Typically, PONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and the premises of the customers. The “first mile” is generally a logical point-to-multipoint network, where a central office serves a number of customers. For example, a PON can adopt a tree topology, wherein one trunk fiber couples the central office to a passive optical splitter/combiner. Through a number of branch fibers, the passive optical splitter/combiner divides and distributes downstream optical signals to customers and combines upstream optical signals from customers (see FIG. 1). Note that other topologies, such as ring and mesh topologies, are also possible.
Transmissions within a PON are typically performed between an optical line terminal (OLT) and optical network units (ONUs). The OLT generally resides in the central office and couples the optical access network to a metro backbone, which can be an external network belonging to, for example, an Internet service provider (ISP) or a local exchange carrier. The ONU can reside in the residence of the customer and couples to the customer's own home network through a customer-premises equipment (CPE).
In the example of an Ethernet PON (EPON), communications can include downstream traffic and upstream traffic. In the following description, “downstream” refers to the direction from an OLT to one or more ONUs, and “upstream” refers to the direction from an ONU to the OLT. In the downstream direction, because of the broadcast nature of the 1×N passive optical coupler, data packets are broadcast by the OLT to all ONUs and are selectively extracted by their destination ONUs. Moreover, each ONU is assigned one or more Logical Link Identifiers (LLIDs), and a data packet transmitted by the OLT typically specifies an LLID of the destination ONU. In the upstream direction, the ONUs need to share channel capacity and resources, because there is only one link coupling the passive optical coupler to the OLT.
FIG. 1 illustrates a passive optical network including a central office and a number of customers coupled through optical fibers and a passive optical splitter (prior art). A passive optical splitter 102 and optical fibers couple the customers to a central office 101. Passive optical splitter 102 can reside near end-user locations to minimize the initial fiber deployment costs. Central office 101 can couple to an external network 103, such as a metropolitan area network operated by an Internet service provider (ISP). Although FIG. 1 illustrates a tree topology, a PON can also be based on other topologies, such as a logical ring or a logical bus. Note that, although in this disclosure many examples are based on EPONs, embodiments of the present invention are not limited to EPONs and can be applied to a variety of PONs, such as ATM PONs (APONs), and gigabit PONs (GPONs, which are PONs using a variant of a generic framing protocol).
In order to reduce the cost of delivering the broadband service to subscribers, it is desirable for a service provider to deliver service to a great number of subscribers, such as over 500 subscribers, from a single node. In order to deliver service to subscribers in more sparsely populated areas in a cost efficient way, it is desirable to implement a PON that can reach subscribers who are located a long distance, such as over 100 km, away from a central office. However, the passive nature of the PON plus the splitting-induced loss have limited the number of subscribers within a PON, typically 32 or 64, and the distance from the central office to a subscriber (the distance from an OLT to an ONU), typically 20 km.